It is known to tighten a screw, for example, until the yield point is reached. In this case, the torque to be expended is measured, for example, by means of a torque sensor or through the power consumption of the drive motor, and the screwing operation is stopped when a specified limiting value is reached.
When power-impact or pulse drivers are used, however, a torque measurement is very inaccurate. For example, given the pulse-by-pulse torque build-up of the power-impact or pulse driver, the static friction between the screw head and the bearing face must be overcome in each case. This variation between static friction and dynamic sliding friction invalidates the effective torque for tightening the screw. As a result, the actual tightening torque of the screw is virtually unknown and, in addition, the tightening torque is influenced quite heavily by the power-impact and pulse driver. Accordingly, the prestressing forces in the screw also fluctuate quite heavily.